The aim of this project is to determine the detailed molecular structure of the photosynthetic reaction center from the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26 by studying single crystals of this protein complex through x-ray diffraction. Reaction centers are intrinsic membrane proteins that carry out the initial steps in photosynthetic charge separations. The R. sphaeroides reaction center (Mr about 100,000) contains three protein subunits, four bacteriochlorophylls, two bacteriophenophytins, two ubiquinones, and one nonheme iron. We have grown single crystals of the reaction center, which diffract to beyond 3 angstroms resolution, thus making the determination of structure at atomic resolution possible. We plan to determine structure by using conventional crystallographic methods. The R. spheroides reaction center will serve as a model system to study how transmembrane electron transport is coupled to the production of membrane potential. This reaction center has been studied extensively through both spectroscopic and biochemical methods. The reaction center complex is functional in the crystal, therefore we will have a unique opportunity to correlate spectroscopic results with the three-dimensional structure of the complex. This study will contribute to the understanding of the structure of membrane proteins. The R. spharoides reaction center is only one of three membrane proteins for which the availability of highly ordered crystals makes the determination of detailed structure possible. The correlation of its amino acid sequence with the three-dimensional structure will increase the data base for membrane proteins, thereby markedly enhancing structure prediction efforts for these types of proteins.